The production of acetic acid by the carbonylation of methanol in the presence of an iridium catalyst and a promoter such as ruthenium is described, for example, in EP-A-0752406, EP-A-0849248, EP-A-0849249, and EP-A-1002785.
WO-A-95/31426 discloses a process for the production of carboxylic acids or their esters having (n+1) carbon atoms by the liquid phase reaction of carbon monoxide with at least one alcohol having (n) carbon atoms in the presence of a catalytic system based on a compound of iridium and a halogen co-catalyst. The process is characterised by maintaining in the reaction medium water in a volume between greater than 0 and 10%, typically between 0.5 and 8%, preferably between 2 and 8%; the ester corresponding to the carboxylic acid and the alcohol in a volume varying between 2 and 40%; and iodides in soluble form of such a nature that the atomic ratio of the iodides to iridium is between greater than 0 and 10, typically between greater than 0 and 3, preferably between greater than 0 and 1.5. The volume of halogen co-catalyst in the reaction medium is between greater than 0 and 10%; typically between 0.5 and 8%, and preferably between 1 and 6%. Suitable iodides include alkaline earth metal and alkali metal iodides, and specifically lithium iodide. The process of WO-A-95/31426 is otherwise unpromoted.
EP-A-0643034 describes a process for the carbonylation of methanol and/or a reactive derivative thereof in the presence of acetic acid, an iridium catalyst, methyl iodide, at least a finite concentration of water, methyl acetate and a promoter selected from ruthenium and osmium. In EP-A-0643034 it is said that ionic contaminants such as, for example, (a) corrosion metals, particularly nickel, iron and chromium and (b) phosphines or nitrogen-containing compounds or ligands which may quaternise in situ should be kept to a minimum in the liquid reaction composition as these will have an adverse effect on the reaction by generating I- in the liquid reaction composition which has an adverse effect on the reaction rate. Similarly, it is said, contaminants such as alkali metal iodides, for example lithium iodide, should be kept to a minimum.